I had inadvertently removed the screw from the clear cog while repairing a broken wire to the brush roller.
In fact it didn't matter that the screw had been removed ... you can get the cog out of position with the screw in place.
The key to the position lies with the cog having six teeth which mesh with the six teeth on the machine.
It is just a matter of having the left hand tooth on the cog in the left hand slot on the machine ... in this position the lug on the cog lands against the stop at the end of its travel.
I'm sure you will have all been doing this without thinking ... I probably made it sound more worse by pointing out I had removed the screw.